Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Examples of tachyarrhythmias include supraventricular tachycardias such as sinus tachycardia, atrial tachycardia, and atrial fibrillation (AF), and ventricular tachyarrhythmias such as ventricular tachycardia (VT) and ventricular fibrillation (VF). Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death. Atrial fibrillation is not immediately life threatening, but since atrial contraction is lost, the ventricles are not filled to capacity before systole which reduces cardiac output. This may cause lightheadedness or fainting in some individuals, as well as fatigue and shortness of breath, hindering the individual from carrying out normal daily activities. If atrial fibrillation remains untreated for long periods of time, it can also cause blood to clot in the left atrium, possibly forming emboli and placing patients at risk for stroke.
Cardioversion (an electrical shock delivered to the heart synchronously with an intrinsic depolarization) and defibrillation (an electrical shock delivered without such synchronization) can be used to terminate most tachyarrhythmias, including AF, VT, and VF. As used herein, the term defibrillation should be taken to mean an electrical shock delivered either synchronously or not in order to terminate a fibrillation. In electrical defibrillation, a current depolarizes a critical mass of myocardial cells so that the remaining myocardial cells are not sufficient to sustain the fibrillation. The electric shock may thus terminate the tachyarrhythmia by depolarizing excitable myocardium, which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci.
Implantable cardioverter/defibrillators (ICDs) provide electro-therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. The ICD is a computerized device containing a pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the impulses from the pulse generator. Typically, the leads have electrically conductive coils along their length that act as electrodes. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, by delivering a shock pulse that impresses an electric field between the electrodes to which the pulse generator terminals are connected. The electric field vector applied to the heart is determined by the magnitude of the voltage pulse and the physical arrangement of the shocking electrodes, which may serve to concentrate the field in a particular region of the heart. Thus, the particular electrode arrangement used will dictate how much depolarizing current is necessary in order to terminate a given tachyarrhythmia.
Ventricular and atrial fibrillation are phenomena that exhibit a threshold with respect to the shock magnitude and duration needed to terminate the fibrillation by changing the transmembrane potential in a critical mass of myocardial cells. The ventricular defibrillation threshold (VDFT), for example, is the smallest amount of energy that can be delivered to the heart to reliably convert ventricular fibrillation to normal sinus rhythm. Similarly, the atrial defibrillation threshold (ADFT) is the threshold amount of energy that will terminate an atrial fibrillation. The larger the magnitude of the shocks delivered by an ICD, the more the battery is drained, thus decreasing the longevity of the device. It is desirable, therefore, for the defibrillation threshold to be as small as possible in order to minimize the amount of shocking current that the ICD must deliver in order to terminate a given tachyarrhythmia.
Electrode arrangements have been devised in an attempt to minimize the defibrillation threshold for particular types of tachyarrhythmias. For example, the traditional configuration for ventricular defibrillation is to place a cathodic electrode in the right ventricle, with the anode formed jointly by an electrode placed in the superior vena cava and the conductive housing of the ICD acting as an additional electrode. For treating atrial fibrillation, a conventional electrode configuration is to use electrodes disposed within the coronary sinus and in the right atrium. In addition, the waveform of the shocking pulse also affects the defibrillation threshold. ICDs use a capacitor discharge system for delivering shock pulses in which a charged capacitor is connected to the shock electrodes to deliver current to the myocardium. Because of space constraints, the size of a typical capacitor is limited and thus exhibits a significant exponential decay when connected to the load (i.e., a small RC time constant). Rather than allowing the decay to continue when the capacitor is connected across the load, solid-state switches may be used to sharply truncate the waveform which may result in a lower energy requirement for defibrillation. ICDs also commonly employ a biphasic shock pulse waveform in which the polarity of the waveform reverses during the shock pulse, a technique that has been found to further lower the defibrillation threshold. (See U.S. Pat. No. 4,998,531, hereby incorporated by reference.)
In order to further improve safety and avoid unnecessary discomfort for ICD patients, there is a continuing need for methods and apparatus that improve the efficiency of electrical defibrillation and thereby reduce the defibrillation threshold. Such reductions in defibrillation thresholds may also expand the population of patients for whom ICDs are an appropriate therapeutic option. It is toward this general objective that the present invention is directed.